Historically, photoillumination techniques have been prominently featured in the main stream of evolving process technology for post-chlorinating vinyl chloride polymers. In fact, a primary interest in photochemical chlorination for post-chlorination of polyvinyl chloride resins has persisted over the years in spite of gradually emerging evidence of certain advantages in conducting post-chlorinations on such resins while in a solid granular state rather than in a solution or highly solvent swollen gel form.
The strong emphasis on actinic light or other forms of photochemical activation in the development of this art is readily seen in the following representative set of early patents:
U.S. Pat. No. 2,426,080--PVC resin in solution. PA0 U.S. Pat. No. 2,590,652--Dry powdered resin in rotary reactor. PA0 U.S. Pat. No. 2,996,489--Aqueous suspension of granular resin containing some solvent. PA0 British Pat. No. 1,120,796--PVC resin dispersed in hydrochloric acid purged to remove oxygen before introduction of chlorine gas. PA0 British Pat. No. 1,200,711--Dry granular resin impregnated with minor proportion of solvent. PA0 U.S. Pat. No. 3,700,632--Fluid bed of dry granular resin. PA0 British Pat. No. 1,378,364--Resin in form of 30 to 300 micron-sized coagula of suspension polymerized PVC powder (either dry or in aqueous suspension). PA0 U.S. Pat. No. 4,049,517--Aqueous suspension of vinyl chloride resin in absence of any solvent. PA0 U.S. Pat. No. 3,100,762 (Shockney)--teaches that in the substantial absence of both actinic radiation and oxygen, an aqueous suspension of porous, granular PVC resin can be successfully chlorinated in the presence of a chloromethane swelling again at temperatures of 60.degree. to 100.degree. C. and gauge pressures of from about 20 to about 80 psi. PA0 U.S. Pat. No. 3,287,336 (Taylor)--demonstrates the feasibility of effecting post-chlorination of granular PVC resin dispersed in aqueous media at temperatures of 54.degree. to 85.degree. C. by activation with a small amount of oxygen rather than by means of actinic radiation. PA0 U.S. Pat. No. 3,328,371 (Beer)--recommends use of an acyl persulfonate catalyst in the absence of light to activate the post-chlorination of PVC polymers dispersed either in a chlorinated hydrocarbon or in an aqueous medium in the presence of a chlorinated hydrocarbon. PA0 U.S. Pat. No. 3,506,637 (Makino)--indicates that photo-radiation and elevated pressures are undesirable constraints and recommends use of temperatures from 55.degree. to 80.degree. C. and use of chlorine gas containing 0.05 to 0.35 percent oxygen by volume for chlorinating PVC resin agglomerates while dispersed in water or hydrochloric acid free of swelling agents.
Photochemical activation of post-chlorination processes for vinyl chloride polymers has continued to occupy a prominent role in the technical literature of the intervening period and the present era as well, as witnessed by the following patents:
Some negative, or at least neutral, comments concerning the use of photoactivated post-chlorination of PVC resins are also encountered in the patent literature, as illustrated by the following tabulation of four specific noteworthy examples.
The last four patents highlighted above appear to offer promising alternatives to the heavy reliance usually placed upon photochemical activation techniques in achieving satisfactory chlorination of granular PVC resins. However, the processes proposed by Shockney and by Beer both require the presence of substantial proportions of extraneous chlorinated hydrocarbon swelling agents (such as the chloromethanes) which must then be removed from the chlorinated PVC resin product and recovered. The other two of the above four patents (namely Taylor and Makino) both depend upon careful regulation of the proportion of molecular oxygen supplied to their reaction systems while the PVC resin is being chlorinated. In view of the low concentrations of oxygen involved, maintaining complete and accurate control over the quantitative supply of same throughout the chlorination process appears to be just as difficult a regulatory method to rely upon as those involving attempts to apply complicated photo-radiation devices uniformly on a large scale.
In view of this state of the art, there continues to be a need for simpler and more straightforward methods of carrying out the chlorination of granular vinyl chloride resins on a commercial scale wherein tight control can easily and reliably be maintained over the course of the chlorination reaction throughout the process, thereby insuring consistent production of chlorinated vinyl chloride resins of high quality. Accordingly, major goals in devising the process of the present invention are the elimination of superfluous components and/or potentially confusing or complicating influences whenever possible, e.g., extraneous swelling agents, molecular oxygen, actinic radiation, etc.